1. Field of the Invention
This invention relates to cryosurgical instruments, and more particularly to vacuum reservoir cryosurgical instruments.
2. Description of the Prior Art
The first hand held cryosurgical instrument for general surgical use is set forth in my U.S. Pat. No. 3,534,739. That instrument controlled the venting of continuously, self-boiling cryogen in order to force the delivery of liquid cryogen as desired. Since the device is continuously vented, except against the pressure induced by the operator in manipulating the vent closing valve thereof, no pressure relief was necessary, and none of the liquid cryogen flowed through the valve. As clinical history developed in the years following the introduction of this hand held cryosurgical unit, more and more usage was found therefore.
In attempts to provide lower cost cryosurgical units which would utilize the liquid cryogen more economically by reducing the rate of boiling thereof while in the reservoir of a cryosurgical instrument, standard, commercial vacuum bottles having dual metallic walls have been used as the basis for construction of cryosurgical instruments. These devices, it has been found, have numerous drawbacks. First of all, since the intent is to conserve nitrogen, the devices were not provided with normally open vents, the closing of which would force liquid through the delivery tube, but rather were provided with valves directly in the cryogen delivery tube, with pressure relief valves to avoid the buildup of excess pressure therein. This, in turn, causes the units to be normally operated at some nominal pressure (such as 10 or 15 psi) which means that the unit is quiescently under pressure even when not being used so long as there is cryogen in the reservoir. Additionally, the rupture or breach of the internal wall of the vacuum bottle has been found to allow cryogen to leak into the normally evacuated space between the walls, causing it to vaporize and expand. Typically, these vacuum bottles have pinched tubes at the bases thereof through which the vacuum is pulled prior to sealing them off. Normally, the pinched tube is open whenever the interwall space is pressurized by cryogen. Naturally, this type of problem is one which is not normally faced in the manufacture of commercial vacuum bottles for ordinary domestic use. An additional problem is that should the pinched tube not prove to be the weakest point in the interwall cavity, the chances are that the joint between the two walls near the mouth of the bottle will be the weakest point. Should this joint rupture while the instrument is in use, it could cause the control and delivery portion of the device to be explosively expelled from the reservoir, and could even cause direct spraying of liquid or gaseous cryogen on the user or the patient being treated.